[ROSIDL] Leverage rosidl_parser.parser to parse .idl directly (#1401)

This PR switches the ROSIDL generation path to parse `.idl` files directly via `rosidl_parser.parser`, removing the intermediate `.idl -> .msg/.srv/.action` conversion step and updating tests to account for IDL-specific type differences (notably `char`).

**Changes:**
- Add a new Python-based `.idl` parser (`rosidl_parser/idl_parser.py`) and expose it via `parseIdlFile()` in `rosidl_parser/rosidl_parser.js`.
- Update the JS generator to generate message/service/action structs directly from parsed `.idl` specs, and remove the old `rosidl_convertor` pipeline.
- Adjust several tests to handle `char` being treated as `uint8` in IDL mode (`--idl`).

Fix: #1402

Author: minggangw